Bigger, Stronger but Not Faster: ontogenetic change in the jaw biomechanics of the great sculpin, Myoxocephalus polyacanthocephalus

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Suction feeding is the most common vertebrate feeding mode. Fishes suction feed by rapidly expanding the buccal cavity, creating a subambient pressure inside the mouth that causes water (and, ideally, a prey item) to rush in. The predator’s ability to close the mouth around evasive prey determines feeding success. As a fish grows, the volume it engulfs should scale with length to the third power (volume ∝ length3). This becomes a burden on larger fishes, as muscle force (which drives mouth closing) should scale with length squared (force ∝ muscle cross-sectional area ∝ length2). Since suction volume increases faster with size than muscle force, a force deficit results as fish grow larger. Two ways to counteract this deficit are to increase muscle mass or increase skeletal leverage within the jaw. In this study, we examined musculoskeletal variation in anatomy and kinematics across an ontogenetic series in the suction-feeding great sculpin, Myoxocephalus polyacanthocephalus. Our results show that great sculpin mandibles change shape as they grow, increasing jaw-closing muscle leverage, which counters the force deficit (N = 6, p = 0.0456). Kinematic results agree: a given amount of muscle strain produces less jaw displacement in larger fish (N = 6, p > 0.00015). We did not find disproportionate changes in muscle mass with size (N = 7, p=.514). Smaller fish, therefore, rely on high-velocity jaw closing whereas larger fish rely more on high forces to close the jaw. We hypothesize that a smaller fish needs high speed to reduce the risk of prey escape from a small suction volume, whereas a large fish needs high forces to move the disproportionately large volume of water.